because most sane people prefer the taste of prego/ragu or a homemade spaghetti sauce over the taste of ketchup otherwise they would just op't for the can of speghettio's
Using ketchup as a replacement for spaghetti/pizza sauce or even BBQ sauce is just a abomination to the taste-buds

It would be like me ordering a chef salad and requesting it is made with spam instead of ham